High frequency coupling circuits



.5 75 I ion lVftwor/i Smaentor EDudley G'oodale,

(Ittomeg M KM Sept. 16, 1941. E. D. GOODALE HIGH FREQUENCY COUPLING CIRCUITS Filed June 1, 1940 Patented Sept. 16, 1941 HIGH FREQUENCY COUPLING CIRCUITS E. Dudley Goodale, Flushing, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application Junel, 1940, Serial No. 338,343

Claims.

My present invention relates to improvements in coupling circuits generally adapted to transfer high frequency signal voltages from unbalanced to balanced lines, or from balanced to unbalanced lines.

There have been disclosed in application Serial No. 279,686 filed on June 17, 1939 by E. D. Goodale and R. A. Monfort, and in application Serial No. 315,313 filed on January 24, 1940 by E. D. Goodale, various types of coupling networks adapted to connect balanced and unbalanced lines.

One of the main objects of my present invention is to provide further coupling arrangements for transferring high frequency signal voltage between balanced and unbalanced lines, there being utilized in the present arrangements a novel method of securing operating bias for the coupling tubes.

Other objects of the invention are generally to improve coupling circuits of the types referred to above, and more especially to provide. coupling networks of the cathode-coupled type which are reliable and efiicient in operation.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawing:

Fig. 1 shows a circuit adapted to couple an unbalanced line to a pair of balanced lines,

Fig. 2 illustrates a modification of the coupling arrangement of Fig. 1,

Fig. 3 shows an arrangement for coupling a balanced system into an unbalanced line,

Fig. 4 illustrates a modification of the arrangement of Fig. 3.

Referring now to the accompanying drawing, wherein like reference characters in the various figures designate similar circuit elements, there is shown in Fig. 1 a coupling network which is generally similar to that disclosed in Fig. 1 of the aforesaid Goodale and Monfort application. Referring specifically to present Fig. 1 there. are shown a pair of electron discharge tubes I and 2 which are included in the coupling network adapted to provide the coupling circuit between an unbalanced line and a pair of balanced lines. The unbalanced line is not specifically shown, but it may be, for example, a co-axial cable whose inner lead is at a positive potential with respect to the grounded outer conductor. The inner lead in that case will be connected toa source of signal energy of any desired frequency. For example, the signal energy source may be in a signal range of 60 cycles to 8 megacycles; the signals may be included in the video or normal sound broadcast range. Regardless of the frequency of the signal energy, or the frequency band width of the signal energy, the conductor feeding the first tube I is shown by way of illustration as having a positive alternating current voltage with respect to ground.

It is for this reason that the term unbalanced is applied to the input circuit of tube I. Assuming now that it is desired to transmit the signal energy from the line feeding tube I to a distribution line comprising a pair of leads 5 and 6, attention is directed to the fact that the leads 5 and 6 are balanced. In other words, at any given instant the leads 5 and 6 are of opposite polarity with respect t ground. Such a distribution line, for example, may be a twisted pair of leads adapted to transmit a wide band of television signal energy through an apartment house, or the like, to one or more television receivers. It is to be clearly understood, however, that the specific example shown in Fig. l'involving the transmission of high frequency signal energy from an unbalanced line to a balanced distribution line is merely illustrative. The unbalanced and balanced circuits may be of any other specific type. For example, the unbalanced circuit may be consisted of an iconoscope, whereas the balanced circuit may comprise a push-pull picture signal amplifier. Those skilled in the art will readily perceive other uses for the unbalanced and balanced circuits respectively.

Considering the specific details of the coupling tubes and their associated circuits, the control grid I of tube I is connected to the input lead 3 through a coupling condenser 8. The grid side of condenser 8 is connected to ground through the grid leak resistor 9. The cathode IQ of tube I is connected to ground through a cathode load resistor II and a biasing resistor I2. The plate I2 of tube I is connected to a source of positive potential through an impedance, an intermediate point of the impedance being by-passed to ground through, the high frequency by-pass condenser I3. The section of the impedance between the 'to grid I in degenerative phase.

resistor I2. There is signal frequency voltage developed across each of resistors I I and I2. The

voltage developed across resistor II is applied to control grid I in degenerative phase to the signal frequency voltage applied through condenser 8.-

The signal voltage developed across resistor I2, as will be shown at a later point, is balanced out by virtue of the fact that the resistor I2 is also included in the space current path of the succeeding tube 2. As a result of the degenerative feedback to tube I the impedance across resistor II is low. The signal frequency voltage develnegative bias path between grid I5 and the grounded end of resistor I2. The plate of tube 2 is energized from a source of direct current which is not shown. v The cathode end of'resistor IIis connected to lead 5. The unbypassed resistor II develops signal frequency voltage thereacross which is applied At any given instant the polarities of leads 5 and 6 are opposite with respect to ground, since lead 5 is connected to the cathode end of resistor I1 and lead 5 is connected 'to the cathode end-of resistor I I; In Fig. l there-is shown byway of illustration that when lead 3- is positive relative to ground, thenlead B-is positive and lead 5' is negative with respect to ground. A constant signal potential level exists at'the grid 1- of tube I.

but reversed in phase; Consequently there is a balanced output at tubes I and Let a low impedance and'of low direct current potential suitable for direct connection to'the-transmission lines. Y

By'utilizing the common biasing resistor I2 in series with each of resistors II and IT there are secured additional advantages over the same circuit when not using the common bias resistor I 2. First of all the stability of the coupling network is improved, and the balance existing between leads 5 and 6 is moreaccurately attained. Secondly, it is possible'to obtain the accurate operating negative bias for each of tubes I and 2, since prop-er choice of the magnitude of resistor I2 will simultaneously result the proper provision of the operating bias for each of the tubes. Thirdly. the signal voltage developed across resistor I2 by virtue of the space current flow therethrough from each of tubes I and 2 will balance out; hence resistor I2 will provide no degenerative feedback to eitherof the tubes.

The circuit arrangement of Fig. 2 possesses the advantages outlined in connection with thecircuit arrangement of Fig. l,- but,'in addition, is

satisfactory for transmission of lowfrequencies, and operates well with a low impedance feed.

In this circuit arrangementthe unbalanced line 3 is connected to the control grid of tube I, but

the; output electrodes of; the latter are coupled to apairof electrondischar'ge tubes prior to the balanced leads 5 and 6. Hence, tube 20 has its control grid coupled to the plate of tube I through a coupling condenser 2|, while the cathode of tube 20 is connected to ground through a path which includes the cathode resistor 22 and the bias resistor 23. The lead 5 is coupled to the cathode end of resistor 22. The cathode of tube 30 is connected to ground through the cathode load resistor 3| and the bias resistor 23. The control grid of tube 30 is connected to the cathode end of the cathode load resistor 32 through the coupling condenser 33. Each of the plates of tubes I, 20 and 30 is connected to a source of direct current (not shown), and each of the control grids of these tubes is connected to ground through a grid leak resistorof the proper magnitude. The lead 6 is connected to the cathode end of load resistor 3|.

As explained in connection with Fig. l the signal voltage developed across resistor 32, due to the fact that resistor 32 is unbypassed, is applied in degenerative phase to the control grid of tube I thereby lowering the impedance across the cathode resistor. In the same way there is degenerative feedback of signal Voltage to the grid of tube 20 by virtue of the signal voltage developed across resistor 22. In the same way the signal voltage developed acrossresistor 3i is degeneratively applied to the control grid of tube 30. As in the case of Fig. 1 there is no degenerative feedback due to the common bias resistor 23, because the signal voltages developed across resistor 23, due to each of tubes 30 and 25, is balanced out since resistor 23 is in the space current path of each of these tubes and the signal voltages developed across this common resistor by the space current flow of both tubes are in opposite polarity sense. The output voltages at the plate and cathode of tube I must be equal.

Fig. 3 illustrates the coupling network of Fig. 1 utilized to couple a pair of balanced lines to an unbalanced line. Such a network could well be utilized to couple the'leads 5 and E of a system as shown in Fig. l to anunbalanced line output. In this case tube 43 has its grid connected to ground, while the cathode of tube 40 is connected to ground through a path which includes'resistor M and the common bias resistor 42. The

plate of tube 40 is connected to a direct current source, while it is coupled to the grid of the following tube 43 through a coupling condenser 44. The plate circuit impedance of tube 43 is so adjusted that voltages developed across resistor 42 by signals on lines 5' and '6 are equal. The cathode of tube 43 is connected to ground through a path which includes a resistor 45 in series with the common bias resistor 42. The plate of tube 43 may then be connected to a direct current source, and also coupled through the condenser 46 to the unbalanced output lead 41. One of the leads of the balanced input line is designated by the numeral 5' and is connected tothe cathode end of load resistor 4I, while the other lead 6' is connected to the cathode end of cathode resistor 45. As in the case of Fig. 1 each of re-' In this case there is utilized four electron discharge tubes. The first pair of tubes 60 and have their cathodes connected by a resistor BI, and the midpoint of the latter is connected to ground through a resistor 62. In other words, each half of the unby-passed resistor develops a signal voltage which is degeneratively applied to its associated control grid while the common resistor section 62 functions to provide the operating direct current bias, signal voltages thereacross being cancelled out. The control grid of each of tubes 60 and 10 is connected to ground through a grid leak resistor, and the plate of each of the tubes is connected to a source of direct current.

The succeeding pair of tubes 80 and 90 have their plates connected in common to the positive terminal of a direct current source through the resistor 81, and the unbalanced line 41 is connected in common to the plates through the coupling condenser 82. The cathodes of tubes 80 and 90 are connected together to the common resistor IUD whose midpoint is grounded. The control grids of tubes 80 and 90 are connected to ground by grid leak resistors. The control grid of tube 80 is connected by the coupling condenser 91 to the plate of tube 60, and the control grid of tube 90 is connected by condenser 92 to the end of resistor 6| connected to the cathode of tube 10. By virtue of these connections to the control grids of tubes 80 and 90 each of these grids will be swung negative when leads 5' and 6 have the polarities noted in Fig. 4. However, the unbalanced lead 41 will have a positive polarity with respect to ground when each of the grids of tubes 80 and 90 is negative.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In combination with a source of unbalanced high frequency signal voltage, a pair of electron discharge tubes arranged in cascade, means for impressing signal voltage upon the input grid of the first of said tubes, a resistive impedance connected to, and individual to, the cathode of each tube, a single auxiliary resistive impedance common to, and arranged in series with, each of the first named resistive impedances, said auxiliary impedance being in the space current path ofeach of said tubes, and an output connection to the cathode end of each of said first resistive impedances whereby the signal voltages at said output connections are balanced.

2. In combination with a source of unbalanced high frequency signal voltage, a pair of electron discharge tubes arranged in cascade, means for impressing signal voltage upon the input grid of the first of said tubes, a resistive impedance connected to, and individual to, the cathode of each tube, a single auxiliary resistive impedance common to, and arranged in series with, each of the first named resistive impedances, said auxiliary impedance being in the space current path of each of said tubes, and an output connection to the cathode end of each of said first resistive impedances whereby the signal voltages at said output connections are balanced, and each of said first named resistive impedances and said auxiliary impedance being unby-passed for high frequency signal voltages.

3. In a coupling network between an unbalanced signal voltage source and a balanced voltage network, a pair of cascaded tubes, means for impressing the unbalanced signal voltage between the input electrodes of the first tube, means for impressing an output voltage of the first tube upon the input electrodes of the second tube, a resistive impedance individual to the cathode of each of said tubes and being in the space current path of its respective tube, an auxiliary resistor common to, and arranged in series with, each of said resistive impedances and being traversed by the space currents of both of said tubes, and an output connection to the cathode end of each of said resistive impedances.

4. In combination, a pair of cascaded electron discharge tubes each being provided with a control grid, a cathode and a plate, a resistive impedance individual to each of said tubes and being connected between the cathode of its respective tube and ground, each of said resistive impedances being unby-passed, a bias resistor common to said impedances and arranged in series between each of said resistive impedances and ground and being traversed by the space current of each of said tubes, means for applying signal voltage developed across each of said resistive impedances in degenerative phase to the control grid of its respective tube, and direct current voltage developed across said bias resistor being applied to each of said control grids, means for applying high frequency signal voltage to the control grid of the first of said tubes, and a high frequency output connection individual to each tube and connected to the cathode end of its respective resistive impedance.

5. In combination, a pair of electron discharge tubes each being provided with a control grid, a cathode and a plate, a resistive impedance individual to each of said tubes and being connected between the cathode of its respective tube and ground, each of said resistive impedances being unby-passed, a common bias resistor arranged inresistive impedance, said signal voltage means including an unbalanced line, a third tube provided with a cathode load element, a connection between the plate of the third tube and the control grid of one of said pair of tubes, and a second connection between the cathode end of said load element and the control grid of the second of said pair of tubes.

E. DUDLEY GOODALE. 

